This invention relates to a device and a method for accurately and quickly measuring the heat output of an electric heat lamp associated with an optical thermoluminescent dosimeter (TLD) reader.
Optical TLD readers are used in the nuclear power industry to measure radiation by using the infrared output of an electric lamp to heat a radiation badge coated with thermoluminescent (TL) phosphors. The badge is worn by personnel working in radiation areas. When the badge is exposed to radiation, electrons in the TL phosphors are energized to an excited state. When these exposed badges are heated in the TLD reader, energy stored in the phosphors is liberated in the form of photons as the electrons return to their ground state. These photons are then counted in the TLD reader and converted to a radiation measurement.
The preferred embodiment of this invention is used with a Panasonic TLD reader. This reader is adapted to read radiation badges containing two types of phosphors: calcium sulfate doped with thulium (CaSO.sub.4 :Tm) and lithium borate doped with copper (Li.sub.2 BO.sub.4 O.sub.7 :Cu). Other phosphors may be used depending on the application. It is a property of these phosphors that some electrons are trapped at relatively low energy levels and are released and return to their ground state within a relatively short period of time due to ambient temperature. This phenomenon is known as fading. Because the badges are read at varied times after irradiation, fading must be correctly accounted for at the time of badge reading to insure an accurate reading. Another property of these phosphors that must be accounted for during reading is the fact that at high temperatures the phosphors radiate in the infrared. This phenomenon is known as thermal noise and should be maintained at a relatively low level to insure an accurate radiation reading.
Proper temperature control during reading is critical to obtaining an accurate radiation reading. Too high a temperature can result in too much thermal noise as well as improper fading. Too low a temperature can result in improper fading. In either case, erroneous radiation readings may result.
To better understand this invention, it is helpful to understand the operation of an optical TLD reader. A typical reader contains the following elements: an optical heat source in the form of an electric lamp and a silicon filter; means for supporting a badge during reading; and means for collecting and counting photons from the badge and converting that count to a radiation reading. The collecting means generally includes a photomultiplier tube. The operator places an exposed badge in the reader with the phosphor coating facing toward the photomultiplier tube and away from the heat lamp such that the infrared output of the lamp will strike the rear of the substrate on which the phosphors are mounted. The heat source is specially designed for this application and comprises a convergence cone for focusing the output of the lamp and the silicon filter for transmitting infrared radiation onto the rear of the substrate.
To obtain an accurate radiation reading, the lamp is generally flashed twice. The lamp is generally flashed a third time to prepare the badge to be reused. The first flash, called a preheat flash, liberates electrons in low temperature traps. This preheat flash lasts for approximately 70 milliseconds (msec). Because of time delays inherent in heat transfer, phosphor temperature does not rise instantaneously but continues to rise for about 500 msec. At that point in time, the second reading flash liberates electrons in the high temperature traps which are used by the reader to measure radiation. The reading flash typically has a duration of approximately 100 msec. After a delay of approximately another 500 msec, the lamp is flashed a third time in an annealing or post-heating flash. This flash is not required for radiation reading but prepares the TL elements for reuse. Overall, the temperature of the phosphors rises by about 300.degree. C. in approximately 1.5 seconds.
The timing of the various flashes, as well as the length of these flashes and the heat output of the lamp, are critical for obtaining an accurate radiation reading. For instance, if too little heat is available during the preheat cycle, not all of the electrons in the low temperature traps will be liberated at that time but will be liberated during the reading flash. This may result in an elevated radiation reading. Similarly, too much heat may liberate electrons stored in the high temperature traps during the preheat cycle, resulting in an erroneously low radiation reading. Further, too much thermal noise may be generated during the reading cycle, resulting in an inaccurate reading. Too little heat during the annealing flash may not restore the phosphors for reuse, while too much heat may damage the substrate.
One method currently used to adjust heat lamp output is to analyze glow curves (light output measured as a function of phosphor temperature over time). The drawback of glow curve analysis is that it may take an undesirably long time, on the order of hours or even days, to adjust heat output of the lamp correctly. This is because badges must be irradiated and read to produce each glow curve. Then after voltage and/or timing adjustments have been made to adjust lamp temperature, new badges must be read until the glow curves look correct. This may take many reading cycles, and glow curve analysis is somewhat subjective. An example of a procedure for adjusting lamp output using glow curves can be found in Plato, P., "Heating Adjustments for the UD-710 Automatic TLD Reader" presented at the 9th Annual TLD International Symposium 253 (June 21, 1990).
Therefore, it is a primary object of the present invention to provide a device and a method for quickly and accurately adjusting the heat output of a lamp used in an optical TLD reader.
It is another object of this invention to quickly determine if such a lamp is faulty.
It is a further object of this invention to provide means for adjusting the heat output of a lamp for an optical TLD reader without requiring exposure of TL phosphors to radiation to make such adjustments.